It has long been known in the art to employ ultrasound to weld or fuse together thermoplastic material. Normally, the ultrasound device consists of a drive unit, a so-called converter, which is connected to an a.c. current source and which is disposed to convert the electric oscillation into a mechanical oscillation in the ultrasound unit, and a horn which transfers the oscillation to the material which is to be fused together. As one example of an ultrasound device of this type, mention might be made of U.S. Pat. No. 3,671,366. Occasionally, a so-called booster is mounted between the horn and the converter in order that the desired oscillation picture be obtained.
EP-A1-0 615 907 schematically describes the principle of an ultrasound sealing apparatus. In this publication, a description is further given of a specific variation of an ultrasound sealing apparatus which is very compact in relation to the generally described design with converter, booster and horn. The ultrasound sealing apparatus according to this embodiment comprises an ultrasound unit which includes a drive unit, a so-called horn and a number of reaction bodies. The horn is intended to abut, with a sealing surface formed in its end, against, for example, a packaging material which is to be sealed and clamped with a sealing force against a counter abutment. The drive unit is connected to an a.c. current source and is disposed to convert the electric oscillation into a mechanical oscillation in the ultrasound unit. The horn and the reaction bodies are located on either side of a nominal nodal plane and are designed so that the resonance frequency (or inherent frequency) which is used as the working frequency will give rise to axial oscillations of large amplitude in the sealing surface of the horn and no amplitude, respectively, at the centrally located nodal plane. The ultrasound unit is secured in relation to other machine parts at the nominal nodal plane.
Within the packaging industry, use is normally made of ultrasound sealing in order fuse together two layers of plastic-coated paper laminate for forming a liquid-tight joint. The desired fusion of the material is achieved in that the material which is clamped between the sealing surface and the counter abutment is subjected to a pulsating compression, which, because of hysteresis losses, gives rise to inner heating of the material which in turn results in the material in question partly melting and being fused together. In so-called roller-fed systems, a paper tube filled with product such as milk, juice or the like is clamped together in the transverse direction so that the tube is formed into closed cushion cartons. In so-called sheet-fed systems, an open tube end is flattened and clamped together approximately as the end of a toothpaste tube or the like. These techniques are generally well-known to a person skilled in the art and will not be described in greater detail here.
FIG. 1 of the accompanying Drawings shows an ultrasound horn which is intended to be connected to a drive unit and possibly a booster in a conventional manner (not shown). The ultrasound horn 1 has a sealing surface 2 of a width B which is slightly greater than the width of the joint which is to be created. In order to obtain a unitary amplitude in the axial A oscillations of the sealing surface 2 along the entire width of the sealing surface 2, the ultrasound horn 1 is provided with two axially extending recesses 3, 4.
An ultrasound horn 1 cannot be made infinitely wide, but it is necessary to take into account the transversely directed oscillations which occur because of the transverse contraction of the material (Poissons number). If a horn is made excessively wide without being provided with recesses, its transverse oscillations, which per se limit the oscillations in the axial direction, result in an excessively great variation in axial amplitude along the sealing surface. If the sealing distance is excessively large for a monolithic cast ultrasound horn of the type which is shown in EP-A1-0 615 907, it is possible either to place a plurality of horns side by side or to make a wide horn with a number of recesses which corresponds to the division into several horns. Using a plurality of horns placed alongside one another is less cost-effective and, moreover, the difficulty is introduced of orienting the horns correctly in relation to one another.
As was explained above, these recesses 3, 4 are thus necessary for correct operation to be obtained for a construction with one single wide horn. The recesses 3, 4 are designed so that they are defined by two parallel lines 3a–b, 4a–b which are located at a distance C from one another and two semi-circles 3c–d, 4c–d at the ends. The semi-circles 3c–d, 4c–d have a radius which is half as large as the above mentioned distance C between the parallel lines.
It has unfortunately proved that, in the ultrasound sealing, the inner tensions in the material reach high levels precisely at these recesses 3, 4, and it is common that the ultrasound horn 1 fails at these recesses 3, 4 because of fatigue cracks which derive from the lower end radii 3c, 4c of the recesses 3, 4. In accordance with general construction and design rules, it has been tested, in order to be able to increase the end radius 3c, 4c in the hope of reducing the levels of tension, to increase the width C of the recesses 3, 4. However, this has proved not to be successful, since an increased width C of the recesses 3, 4 increases the flexural effects at the lower ends 3c, 4c of the recesses 3, 4, An increased width C of the recesses also gives rise to an increased variation in the oscillation amplitude of the sealing surface 2 immediately beneath the recesses.
EP-A2-1 088 760 also discloses various designs of elongate sealing horns provided with axially extending recesses. In this context, it might also be mentioned that there are different configurations of the ultrasound horns as regards their profile seen from the side. The above-mentioned EP-A1-0 615 907 shows an arcuate version, in FIG. 1 there is shown an arcuate design with a right angle upper portion and there are also variations where the arcuate portion in FIG. 1 is instead rectilinear cuneiform (see broken line in FIG. 1). Regardless of the form of the profile, they all suffer from the above-outlined problem as regards fatigue cracks at the ends 3c, 4c of the recesses 3, 4.
Hence, there is today no satisfactory design and construction of ultrasound horns. The variations which are in existence today have different advantages as regards unitary amplitude along the width of the sealing surface, as regards propagation of inherent frequency because of manufacturing tolerances, as regards proximal, undesirable other inherent frequencies etc. However, they also suffer from similar problems in respect of failure at the recesses and other failures at different transitional zones between different parts.